(a). Field of the Invention
The present invention relates in general to an electronic circuit, and more particularly to an amplifying circuit with a variable gain.
(b). Description of the Prior Arts
In modem circuit design, it is often necessary to provide the signal amplifying function with high accuracy. For example, in a common modulation mechanism for wireless communication, I/Q signals are used in phase modulation or frequency modulation. However, as long as the amplitude of the I/Q signals has a slight mismatch, the constellation diagram of the signals would be damaged, and the bit error rate (BER) of the communication system would also be increased. Thus, an amplifier with a highly precise gain is needed to accurately control the amplitude of the I/Q signals.
The gain drift of a conventional open-loop amplifier is rather large, so it cannot meet the requirement of highly precise gain. It is also difficult for a common closed-loop amplifier to precisely control its gain. FIG. 1 is a circuit diagram of a conventional closed-loop amplifier, which includes an operational amplifier (op-amp) 10, a resistor R1 coupled between the input of the whole circuit and the inverting input of the op-amp 10, and a feedback resistor R2 coupled between the output of the whole circuit and the inverting input of the op-amp 10. Vin and Vout are the input voltage and output voltage respectively. Since the properties of negative feedback and “virtual short”, the gain (Vout /Vin) of the amplifying circuit in FIG. 1 can be derived as −R2/R1. By adjusting the resistor values of R1 and R2, a required gain is obtained. However, if a highly precise gain is required, e.g. 1.001, then the gain accuracy may be easily affected by a slight error of the resistor values of R1 and R2 since the difference between the values of R1 and R2 is rather small (e.g. R1=1 k ohm, R2=(1 k+1) ohm). In particular, when the amplifying circuit of FIG. 1 is implemented on an integrated circuit (IC), it is more difficult to achieve a highly precise gain since the factors of temperature, manufacturing process, supply voltage drift, etc. cannot be completely controlled.